1. Field of the Invention
The present invention relates to a method for manufacturing a microstructure, such as a micromirror element, an acceleration sensor element, an angular velocity sensor element, or a vibration element, produced by micromachining technology.
2. Description of the Related Art
Elements having a microscopic structure formed by micromachining technology have been finding application in an increasing number of fields in recent years. For example, there has been interest in microscopic micromirrors having a light reflecting function in the field of optical communications technology.
In optical communications, optical fibers are used as the medium for transmitting optical signals, and what is known as an optical switching device is generally used to switch the transmission path of the optical signals from one fiber to another. Characteristics that are required of optical switching device in order to achieve good optical communications include a large capacity, high speed, and high reliability in the switching operation. Optical switching devices that incorporate micromirror elements produced by micromachining technology are increasingly promising from these standpoints. A micromirror element allows switching between the optical transmission paths on the input and output sides of an optical switching device to be performed completely with optical signals, without first converting an optical signal into an electrical signal, which is preferable in terms of obtaining the above-mentioned characteristics.
A micromirror element is equipped with a mirror surface for reflecting light, and the direction in which the light is reflected can be changed by pivoting this mirror surface. An electrostatic drive type of micromirror element that utilizes static electricity to pivot the mirror surface is employed in many optical devices. These electrostatic micromirror elements can be broadly classified into two types: micromirror elements manufactured by what is known as surface micromachining technology, and micromirror elements manufactured by what is known as bulk micromachining technology.
With surface micromachining, a thin film of material is worked into the desired pattern corresponding to the various structural areas on the substrate, and these patterns are successively laminated to form a support, mirror surface, electrodes, and other such members constituting the element, or sacrificial layers to be removed subsequently. Meanwhile, with bulk micromachining, the material substrate itself is formed into the desired shape for a support, mirror, and so forth, and the mirror surface and electrodes are formed from thin films as necessary. Bulk micromachining is disclosed in JP-A 5-302182, JP-A 10-214978 and JP-A 10-256569, for example.
One of the technological requirements of a micromirror element is that the mirror surface used for light reflection have a high degree of flatness. With surface micromachining, though, because the mirror surface ultimately formed is so thin, the mirror surface is prone to curving, and high flatness cannot be ensured unless the length of the mirror surface on one side is no more than a few dozen microns.
In contrast, with bulk micromachining, a mirror component is produced by etching away the material substrate itself, which is relatively thick, and the mirror surface is provided over this mirror component, so good rigidity can be ensured even with a mirror surface of broader surface area. As a result, it is possible to form a mirror surface having sufficiently high optical flatness. Therefore, bulk micromachining is widely employed in the manufacture of micromirror elements, particularly when the mirror surface needs to be 100 μm or longer on one side.
FIG. 29 is a simplified exploded perspective view of a micromirror element 400, which is an example of a conventional electrostatic drive type of micromirror element produced by bulk micromachining. This micromirror element 400 comprises a mirror substrate 410 and a base substrate 420, which are laminated via a spacer (not shown). The mirror substrate 410 has a mirror component 411, an inner frame 412, and an outer frame 413. The mirror component 411 and the inner frame 412 are linked by a pair of torsion bars 414. The inner frame 412 and the outer frame 413 are linked by a pair of torsion bars 415. The torsion bars 414 define the axis of rotation of the mirror component 411 with respect to the inner frame 412. The torsion bars 415 define the axis of rotation of the inner frame 412, and its attendant mirror component 411, with respect to the outer frame 413.
A pair of plate electrodes 411a and 411b are provided on the back side of the mirror component 411, and a mirror surface (not shown) for reflecting light is provided on the front side. A pair of plate electrodes 412a and 412b are provided on the back side of the inner frame 412.
Plate electrodes 420a and 420b are provided to the base substrate 420 so as to be opposite the plate electrodes 411a and 411b of the mirror component 411, and plate electrodes 420c and 420d are provided so as to be opposite the plate electrodes 412a and 412b of the inner frame 412. With a conventional micromirror element, the drive means most often employed is to generate electrostatic force with plate electrodes such as these.
With a structure such as this, if the plate electrode 420a of the base substrate 420 is negatively charged in a state in which the plate electrode 411a of the mirror component 411 is positively charged, for example, electrostatic force is generated between the plate electrode 411a and the plate electrode 420a, and the mirror component 411 pivots in the direction of arrow M1 while twisting the pair of torsion bars 414.
Meanwhile, if the plate electrode 420c of the base substrate 420 is negatively charged in a state in which the plate electrode 412a of the inner frame 412 is positively charged, for example, electrostatic force is generated between the plate electrode 412a and the plate electrode 420c, and the inner frame 412 pivots along with the mirror component 411 in the direction of mirror M2 while twisting the pair of torsion bars 415. FIG. 30 shows what happens when this rotational drive causes the inner frame 412 and its attendant mirror component 411 to be displaced by an inclination angle θ with respect to the outer frame 413.
The orientation of the plate electrodes 411a and 411b with respect to the plate electrodes 420a and 420b is different in the state shown in FIG. 29 from that in FIG. 30. Accordingly, in the various states shown in FIGS. 29 and 30, even though the same voltage is applied between the plate electrode 411a and the plate electrode 420a, the size of the generated electrostatic force will be different, and as a result the inclination angle of the mirror component 411 with respect to the inner frame 412 will also be different. Therefore, for the inclination angle of the mirror component 411 with respect to the inner frame 412 to be the same in the various states shown in FIGS. 29 and 30, an electrostatic force of suitable size and varying according to the state must be generated between the plate electrode 411a and the plate electrode 420a, for example. To accomplish this, the voltage applied to the plate electrode 411a and the plate electrode 420a must be controlled as dictated by the inclination angle of the inner frame 412 with respect to the outer frame 413.
To control the applied voltage in this way, it is necessary to employ some means such as storing data about the inclination angle corresponding to the voltage applied to the inner frame 412 of the mirror component 411, and data about the inclination angle corresponding to the voltage applied to the outer frame 413, and selecting the applied voltage through reference to these data. This results in a tremendous amount of data. Accordingly, with a micromirror element 400 that employs a drive system that involves applied voltage control such as this, it is difficult to increase the switching speed, and too much burden is imposed on the drive circuit.
With the plate electrode structure employed in the micromirror element 400, the plate electrodes 420a, 420b, 420c, and 420d provided to the base substrate 420 provide drive so that the mirror component 411 equipped with the plate electrodes 411a and 411b and the inner frame 412 equipped with the plate electrodes 412a and 412b are pulled in, so there is a pull-in voltage in this drive. Specifically, this is a phenomenon whereby the mirror component 411 or the inner frame 412 is suddenly pulled in at a certain voltage, which can result in a problem in that the inclination angle of the mirror component 411 cannot be properly controlled. This problem is particularly pronounced when a large inclination angle (about 5° or more) is attempted, that is, when the extent of twisting of the torsion bars is great.
One way that has been proposed for solving this problem is to drive the micromirror element with a comb electrode structure rather than a plate electrode structure. FIG. 31 is simplified exploded perspective view of a conventional micromirror element 500 that makes use of a comb electrode structure.
The micromirror element 500 has a mirror component 510 with a mirror surface (not shown) provided on its upper or lower surface, an inner frame 520, and an outer frame (only partially shown), and comb electrodes are integrally formed are each of these. More specifically, a pair of comb electrodes 510a and 510b are formed on the mirror component 510, extending outward from the pair of parallel sides thereof. A pair of comb electrodes 520a and 520b are formed on the inner frame 520, extending inward and corresponding to the comb electrodes 510a and 510b, and a pair of comb electrodes 520c and 520d are formed extending outward. A pair of comb electrodes 530a and 530b are formed on the outer frame 530, extending inward and corresponding to the comb electrodes 520c and 520d. The mirror component 510 and the inner frame 520 are linked by a pair of torsion bars 540, and the inner frame 520 and the outer frame 530 are linked by a pair of torsion bars 550. The pair of torsion bars 540 define the axis of rotation of the mirror component 510 with respect to the inner frame 520, while the torsion bars 550 define the axis of rotation of the inner frame 520, and its attendant mirror component 510, with respect to the outer frame 530.
With a micromirror element 500 structured in this way, a set of comb electrodes provided close together in order to generate electrostatic force, such as the comb electrode 510a and the comb electrode 520a, are divided into upper and lower levels, as shown in FIG. 32a, when no voltage is being applied. When voltage is applied, as shown in FIG. 32b, the comb electrode 510a is pulled into the comb electrode 520a, which drives the mirror component 510. More specifically, in FIG. 30, if the comb electrode 510a is positively charged and the comb electrode 520a is negatively charged, for example, the mirror component 510 pivots in the direction of M3 while twisting the pair of torsion bars 540. Meanwhile, if the comb electrode 520c is positively charged and the comb electrode 530a is negatively charged, the inner frame 520 pivots in the direction of M4 while twisting the pair of torsion bars 550.
These two rotary operations are independent of one another. Specifically, prior to the application of potential to the comb electrodes 510a and 510b and the comb electrodes 520a and 520b, the comb electrodes 510a and 510b are always in the same state of orientation with respect to the comb electrodes 520a and 520b, regardless of the inclination angle of the inner frame 520 with respect to the outer frame 530. Thus, the inclination angle of the inner frame 520 and its attendant mirror component 510 with respect to the outer frame 530 has no effect in the micromirror element 500, which simplifies control of the inclination angle of the mirror component 510.
Also, when a comb electrode structure is employed, the direction in which the electrostatic force acts is set to be substantially perpendicular to the direction in which the mirror component 510 pivots. Therefore, in the drive of the mirror component 510, contact with the comb electrodes due to pull-in is less apt to occur, and as a result it is possible for the mirror component 510 to have a suitably large inclination angle.
With the micromirror element 500, the combs (electrodes) are displaced as the mirror component 510 and the inner frame 520 rotate, so the comb electrodes must be formed in a suitable thickness appropriate for this inclination angle of the mirror component 510 and the inner frame 520. For instance, if the length D of the body portion 511 of the mirror component 510 is 1 mm, and if the mirror component 510 is inclined by 5° around the axis defined by the pair of torsion bars 540 with respect to the inner frame 520, then one body end 511′ will sink 44 μm. Accordingly, the thickness T of the comb electrodes 510a and 510b formed on the mirror component 510 must be at least 44 μm.
On the other hand, from the standpoint of obtaining a larger inclination angle at a lower applied voltage, it is preferable for the torsion bars 540 and 550 that afford twisting resistance to be formed thinner. With the conventional micromirror element 500, however, the torsion bars 540 and 550 are formed in the same thickness as the material substrate constituting the mirror component 510, the inner frame 520, and the outer frame 530, and as such these torsion bars are quite thick. For example, if the thickness T of the comb electrodes 510a and 510b is designed to be at least 44 μm as mentioned above, the thickness of the torsion bars 540 and 550 ends up being at least 44 μm along with the mirror component 510. When such thick torsion bars 540 and 550 are used, a larger electrostatic force has to be generated between the comb electrodes in order to twist these bars, which means that the drive voltage also has to be higher. Also, in prior art the twisting resistance of the torsion bars 540 and 550 is adjusted by varying the width of the torsion bars 540 and 550, but merely changing the design in the width direction is often inadequate for setting the proper twisting resistance.
Thus, with a microstructure produced by bulk machining technology, there are cases when different thicknesses or heights are required in various structures formed by etching a material substrate. However, forming a thin structure integrally connected to a thick structure with high precision in the thickness was difficult in conventional bulk machining technology.